My starter is about 2 years old and suddenly has been producing bread that had great crumb, spring, crust, etc, but doesn't have the distinctive "sour" taste or smell! I have a recipe that I've perfected and I really don't deviate from it. The starter doesn't even smell as sour as it used to. It seems to be just as active as it always has been - bubbling after being fed, rising the bread, etc. Any ideas?